Catalyst for steam cracking reactions and related preparation process

ABSTRACT

Catalyst for steam cracking reactions consisting of pure mayenite having the general formula:  
     12CaO.7Al 2 O 3    
     having an X-ray diffraction spectrum as indicated in Table I, obtained with a preparation process comprising the following steps:  
     dissolution of salts containing calcium and aluminum with water;  
     complexing of the dissolved salts by means of polyfunctional organic hydroxyacids;  
     drying of the solution resulting from the complexing in order to obtain a solid precursor product;  
     calcination of the solid precursor product at a temperature ranging from 1300 to 1400° C. for at least two hours.

[0001] The present invention relates to a catalyst for steam crackingreactions and the related preparation process.

[0002] The most widely-used method for the production of light olefins,in particular ethylene and propylene, is the steam cracking process, inwhich a hydrocarbon charge is heated, in the presence of water vapor, inspecific ovens to produce a gaseous stream rich in olefins. Steamcracking is a thermal process which is carried out on an industrialscale without catalysts. The setting up of a catalytic system whichallows an increase in the yields to the desired products would provideimportant advantages; owing to the large volumes of products in question(for example the world-wide production of ethylene is over 70Mton/year), even small percentage increases in the yield would have agreat impact on process economy.

[0003] The use of catalysts for steam cracking reactions has not beenwidely studied, even if various companies and research groups haveoccasionally worked in this area since the 70s'. In some cases a processhas been defined but industrial applications are not known at themoment.

[0004] Among the most significant references are the following, whichidentify calcium-aluminate compounds in which the 12CaO.7Al₂O₃(mayenite) phase prevails, as the most active materials for thecatalysis of naphtha cracking:

[0005] A. A. Lemonidou, I. A. Vasalos, Applied Catalysis, 54 (1989),119-138;

[0006] A. A. Lemonidou, I. A. Vasalos, Proc. 1987 AIChE Spring NationalMeeting, Houston, Mar. 29-Apr. 2, 1987;

[0007] K. Kikuchi, T. Tomita, T. Sakamoto, T. Ishida, Chemical &Engineering Progress, 81 (1985) 6, 54.

[0008] B. Basu, D. Kunzru, Industrial & Engineering Chemistry Res.,1992, 31, 146-155.

[0009] Another reference has also demonstrated the good performance ofmaterials consisting of Ca-aluminate mixtures:

[0010] S. Nowak, G. Zimmermann, H. Gushel, K. Anders, in “Catalysis inPetroleum Refining 1989” (D. L. Trimm et al. Eds.), Elsevier SciencePublishers B. V., 1990.

[0011] As far as studies relating to industrial development areconcerned, mention can be made of Asahi Chemical which claims a process,almost ready for commercialization, for steam cracking in a circulatingbed, using a catalyst based on ZSM-5 and ZSM-11 zeolites, charged withmetals such as Fe, Mg and/or Ib metals. This process partially increasesthe yield to ethylene, but the reaction is mainly directed towards theproduction of propylene and aromatics. Recent information (PERP Report96/97S12-Chem Systems, September 1997) reveals that the process stillhas several problems of a technological nature to be solved, among whichmany aspects relating to the catalyst (activity, regeneration,duration), before it can be actually commercialized. More or less thesame situation also applies to the Russian process of Vniios (ResearchInstitute for organic syntheses) which uses potassium vanadate supportedon corindone/mullite as catalyst, with the addition of promoters. Exxonhas patented a process using an inert solid as heat transporter orcatalysts based on mixed oxides of Mg, Ca, Mn, Be, Sr, Ce, V, Cs (W.Serrand et al., WO 97/31083). This process however is preferablydesigned for heavy charges (e.g.>500° C.) and comprises, in fact, aparticular type of horizontal moving bed reactor with two rotatingscrews which help the movement of the charge.

[0012] A technology which seems closer to a possible industrialapplication is the Pyrocat process, set up by Veba Oel and Linde (M.Wyrosteck, M. Rupp, D. Kaufmann, H. Zimmermann, Proc. 15^(th) WorldPetroleum Congress, Beijing, Oct. 12-16, 1997). This technologycomprises implementation of steam cracking plants without modifying thedesign of the ovens. The idea is based on coating the inside of thecracking tubes with a solid layer having a catalytic effect and whichinhibits the formation of coke, thus prolonging the times betweensubsequent stoppages for decoking operations. The catalyst is based onAl₂O₃/CaO and contains, as gasification promoter, compounds of alkalinemetals. The technology however can only be applied to conventionalcracking plants, operating with conventional charges.

[0013] It can therefore be seen from literature that catalysts based oncalcium aluminates can be used in steam cracking reactions for theproduction of ethylene and propylene. The calcium aluminates which canbe formed are the following, in increasing order of calcium content:CaO.6Al₂O₃, CaO.2Al₂O₃, 3CaO.5Al₂O₃, CaO.Al₂O₃, 5CaO.3Al₂O₃,12CaO.7Al₂O₃, 2CaO.Al₂O₃ and 3CaO.Al₂O₃ but it is not disclosed inliterature which is the preferred crystalline phase for steam crackingreactions. In fact, according to Lemonidou (A. A. Lemonidou, I. A.Vasalos, Applied Catalysis, 54 (1989), 119-138) the most effectivecatalyst is a mixture of calcium-aluminates in which the prevalentcompound is mayenite (12CaO.7Al₂O₃); S. Nowak, on the other hand, haspatented a catalyst (DD-243 647 of 1987) in which the preferred phaseshave a lower content of calcium oxide: CaO.Al₂O₃ and CaO.2Al₂O₃.

[0014] The preparation of these catalysts is generally effected by themechanical mixing of the oxides or their aluminum and calcium precursorsand subsequent calcination at a high temperature. This process generallyleads to the formation of materials in which there are several phases,even if in some cases one phase may be distinctly prevalent with respectto the others. No information is provided however in scientificliterature with respect to this type of catalyst, on the production ofpure calcium-aluminate materials by means of the syntheses described.

[0015] We have now found a process for obtaining pure mayenite(12CaO.7Al₂O₃), which surprisingly allows better results to be obtainedin terms of yield to light olefins in the field of naphtha steamcracking reactions with respect to mixtures containing mayenite andother calcium-aluminates either pure or mixed with each other.

[0016] The catalyst for steam cracking reactions, object of the presentinvention, is characterized in that it consists of pure mayenite havingthe general formula:

12CaO.7Al₂O₃

[0017] which has, in its calcined form, an X-ray diffraction spectrum,registered by means of a vertical goniometer equipped with an electronicimpulse count system and using CuKα radiation (λ=1.54178 Å), containingthe main reflections indicated in Table 1 (wherein d indicates theinterplanar distance) and in FIG. 1.

[0018] The process for the preparation of the catalyst, i.e. puremayenite described above, is characterized in that it comprises thefollowing steps:

[0019] dissolution of salts containing calcium and aluminum with water;

[0020] complexing of the dissolved salts by means of polyfunctionalorganic hydroxyacids;

[0021] drying of the solution resulting from the complexing in order toobtain a solid precursor product;

[0022] calcination of the solid precursor product at a temperatureranging from 1300 to 1400° C., preferably ranging from 1330 to 1370° C.,for at least 2 hours, preferably for at least 5 hours.

[0023] The polyfunctional organic hydroxyacids can be selected fromcitric acid, maleic acid, tartaric acid, glycolic acid and lactic acid:citric acid is preferred.

[0024] The salts containing calcium are preferably selected from calciumacetate and calcium nitrate.

[0025] Aluminum nitrate is the preferred salt containing aluminum.

[0026] It is advisable for the preparation process to be carried outwith a molar ratio polyfunctional hydroxyacids/salts containing calciumand alumina ranging from 1.5 to 1.

[0027] A further object of the invention relates to the process for theproduction of light olefins by means of the steam cracking reaction ofhydrocarbon charges selected from naphtha, in particular virgin naphtha,kerosene, atmospheric gas oil, vacuum gas oil, alone or mixed with eachother, in the presence of a catalyst according to claim 1, which iseffected preferably operating at a temperature ranging from 720 to 800°C., at a pressure ranging from 1.1 to 1.8 absolute Atm and for a contacttime ranging from 0.07 to 0.2 sec.

[0028] Some examples are provided for a better illustration of theinvention, but which should not be considered as limiting the scope ofthe present invention.

EXAMPLE 1

[0029] Preparation of the catalyst.

[0030] A synthesis method in homogeneous phase was used.

[0031] This method comprises the use of citric acid or polyfunctionalhydroxyacids which have the function of complexing metal salts inaqueous solution. After dehydration of the aqueous solution an amorphoussolid precursor is obtained, which, after thermal treatment at a hightemperature, produces the desired product.

[0032] The main advantages of this technique are the following:

[0033] homogeneous mixing on an atomic scale

[0034] good stoichiometric control

[0035] production of mixed oxides using commercial chemical products

[0036] short process times

[0037] A solution of aluminum nitrate, 378.2 g of Al(NO₃)₃.9H₂O (1.008moles) in 470 g of water was first added to a solution of calciumacetate, obtained by dissolving at room temperature 152.2 g of(CH₃COO)₂Ca.H₂O (0.864 moles) in 450 g of H₂O, followed by a solution ofcitric acid: 393.1 g (1.872 moles) in 375 g of water. The homogeneoussolution obtained was dried by means of a spray-dryer. The desiredproduct 12CaO.7Al₂O₃ (Mayenite) was obtained in pure form aftercalcination at 1350° C. for 5 h.

[0038] In order to obtain a catalyst formed by means of tableting, alubricating agent (2 wt % of stearic acid) was added; after tableting,the catalyst was subjected to an additional calcination step.

[0039] The composition of the catalyst obtained was verified by means ofX-ray diffractometry, which showed the presence of the single pure12CaO.7Al₂O₃ phase.

[0040] (See Table 1 and FIG. 1 mentioned above).

EXAMPLE 2 (COMPARATIVE)

[0041] In this example the sol-gel method was used.

[0042] 327.62 g of aluminum secbutoxide (1.33 moles) in 327.9 g ofn-butanol (4.431 moles) were charged into a 2 liter three-necked flask.A solution of 200.8 g of (CH₃COO)₂Ca.H₂O (1.14 moles) in 598 g of H₂Owas added, at 80° C., under vigorous magnetic stirring, by means of adrip funnel. The gel formed was left to age for a night and then dried.The composition of the product obtained, after calcination at 1350° C.for 5 h, determined by means of X-ray diffraction, is the following: 21%6 CaO.Al₂O₃, 7% CaO.2Al₂O₃, 3% 3CaO.Al₂O₃, 69% 12CaO.7Al₂O₃.

[0043] In order to obtain a catalyst formed by means of tableting, alubricating agent (2 wt % of stearic acid) was added; after tableting,the catalyst was subjected to an additional calcination step.

[0044] From Table II and FIG. 2 it can be seen that in addition to themayenite phase, there are also the diffraction lines relating to thecalcium-aluminates specified above.

EXAMPLE 3 (COMPARATIVE)

[0045] A solution of aluminum nitrate, 577.7 g of Al(NO₃)₃.9H₂O (1.540moles) in 720 g of water was first added to a solution of calciumacetate, obtained by dissolving at room temperature 67.83 g of(CH₃COO)₂Ca.H₂O (0.385 moles) in 200 g of H₂O, followed by a solution ofcitric acid: 404.3 g (1.925 moles) in 380 g of water. The homogeneoussolution obtained was dried by means of a spray-dryer. The desiredproduct CaO.2Al₂O₃ was obtained in pure form after calcination at 1350°C. for 5 h.

[0046] In order to obtain a catalyst formed by means of tableting, alubricating agent (2 wt % of stearic acid) was added; after forming, thecatalyst was subjected to an additional calcination step.

[0047] The composition of the catalyst obtained was verified by means ofX-ray diffractometry, which showed the presence of the single pureCaO.2Al₂O₃ phase.

EXAMPLES 4-7

[0048] Steam cracking reaction effected in a laboratory plant incontinuous with a fixed bed reactor having a diameter of ½″.

[0049] Operating conditions:

[0050] Charge=Virgin Naphtha

[0051] T=775° C.

[0052] H₂O/charge=0.8 by weight

[0053] Residence time=0.1 s

[0054] 4 tests were effected using the following materials:

[0055] quartz in granules, or inert material as reference for evaluatingthe catalyst performances (Example 4: comparative);

[0056] mixture of calcium aluminates prepared as described in Example 2(Example 5: comparative);

[0057] pure CaO.2Al₂O₃ prepared as described in Example 3 (Example 6:comparative);

[0058] pure mayenite prepared as described in Example 1 (Example 7).

[0059] From the results provided in Table A, it can be seen that all thecalcium-aluminate materials give higher performances than quartz withrespect to yield to C₂, C₃, C₄ olefins (butenes and butadiene); puremayenite (12CaO.7Al₂O₃) however provides the best result, producing thehighest yield, without increasing the formation of undesired productssuch as coke and carbon monoxides. TABLE A Example 5 69% 12CaO · 7Al₂O₃6 7 21% CaO · Al₂O₃ Pure Pure 4 7% CaO · 2Al₂O₃ CaO · 12CaO · Yield (w%) quartz 3% 3CaO · Al₂O₃ 2Al₂O₃ 7Al₂O₃ Hydrogen 0.80 0.98 0.92 0.94Methane 9.78 11.35 10.70 11.14 Ethylene 22.26 25.45 24.90 26.27 Ethane2.23 2.54 2.41 2.52 Propylene 15.12 17.24 15.94 17.60 Propane 0.35 0.410.40 0.48 Butanes 1.31 1.52 1.19 2.04 Butenes 5.77 6.67 5.41 7.35Butadiene 3.96 4.81 3.80 5.03 CO + CO₂ 0.11 0.62 0.16 0.03 Tot. GAS 61.771.6 65.8 73.4 Coke 0.8 0.8 0.7 0.4 Tot. C₂,C₃,C₄ 47.11 54.17 50.0556.25 olefins

[0060] TABLE I X-ray diffraction spectrum of the pure Mayenite phase 2θ(CuKα) (°) d (Å) 18.18 4.88 21.02 4.22 23.52 3.78 27.89 3.196 29.872.989 33.48 2.675 35.17 2.550 36.77 2.442 38.33 2.347 41.31 2.184 44.102.052 46.76 1.941 49.30 1.847 51.76 1.765 52.96 1.728 54.14 1.693 55.301.660 56.44 1.629 57.56 1.600 60.87 1.521 61.95 1.497 62.98 1.475 67.191.392 69.23 1.356

[0061] TABLE II X-ray diffraction spectrum of a sample consisting ofMay- enite (main phase) and Ca₃Al₂O₆, CaAl₂O₄ and CaAl₄O₇. 2θ (CuKα) (°)d (Å) 2θ (CuKα) (°) d (Å) 12.94 6.84 43.19 2.093 14.32 6.18 44.14 2.05016.06 5.52 44.80 2.021 16.40 5.40 45.36 1.998 18.18 4.88 46.39 1.95619.01 4.66 46.81 1.939 19.99 4.44 47.24 1.923 21.02 4.22 47.69 1.90621.98 4.04 48.07 1.891 22.80 3.899 48.85 1.863 23.53 3.777 49.37 1.84423.99 3.707 49.60 1.836 24.71 3.601 50.65 1.801 25.42 3.501 51.83 1.76326.06 3.416 53.02 1.726 27.00 3.299 54.20 1.691 27.92 3.193 55.36 1.65828.24 3.158 56.51 1.627 29.00 3.077 57.63 1.598 29.23 3.053 59.32 1.55729.90 2.986 59.50 1.552 30.10 2.967 60.40 1.531 31.21 2.864 60.96 1.51932.14 2.782 61.99 1.496 32.59 2.745 62.17 1.492 33.22 2.695 63.09 1.47233.52 2.671 63.28 1.468 34.55 2.594 63.82 1.457 35.20 2.548 64.22 1.44935.70 2.513 65.19 1.430 36.83 2.438 65.65 1.421 37.44 2.400 66.44 1.40638.37 2.344 67.29 1.390 38.71 2.324 67.48 1.387 39.75 2.266 68.34 1.37241.02 2.198 69.31 1.355 41.35 2.182 69.55 1.351 42.35 2.133

1. A catalyst for steam cracking reactions characterized in that itconsists of pure mayenite having the general formula: 12CaO.7Al₂O₃ withan X-ray diffraction spectrum as indicated in Table I.
 2. A process forthe preparation of the catalyst according to claim 1 , characterized inthat it comprises the following steps: dissolution of salts containingcalcium and aluminum with water; complexing of the dissolved salts bymeans of polyfunctional organic hydroxyacids; drying of the solutionresulting from the complexing in order to obtain a solid precursorproduct; calcination of the solid precursor product at a temperatureranging from 1300 to 1400° C. for at least two hours.
 3. The processaccording to claim 2 , wherein the calcination is effected at atemperature ranging from 1330 to 1370° C. for at least 5 hours.
 4. Theprocess according to claim 2 , wherein the salts containing calcium areselected from calcium acetate and calcium nitrate.
 5. The processaccording to claim 2 , wherein the salt containing aluminum is aluminumnitrate.
 6. The process according to claim 2 , wherein thepolyfunctional hydroxyacid is citric acid.
 7. The process according toclaim 2 , wherein the molar ratio polyfunctional hydroxyacids/saltscontaining calcium and alumina ranges from 1.5 to
 1. 8. A process forthe production of light olefins by means of the steam cracking reactionof hydrocarbon charges selected from naphtha, kerosene, atmospheric gasoil, vacuum gas oil, alone or mixed with each other, in the presence ofa catalyst according to claim 1 , operating at a temperature rangingfrom 720 to 800° C., at a pressure ranging from 1.1 to 1.8 absolute Ate.and for a contact time ranging from 0.07 to 0.2 sec.
 9. The processaccording to claim 8 , wherein the naphtha is virgin naphtha.